Add Gemfile and .rspec for testing

This commit adds the Gemfile needed to install gems with Bundler in
order used for development and running spec tests. It also adds a
.rspec with some options set for running rspec.
Clean up spec tests for rspec 2.11.3 and up

Starting in Rspec 2.11.3, a deprecation warning was added for
`expect { }.should` syntax. The desired syntax is `expect { }.to`.

This commit update two instances of `expect { }.should` in the
'spec/unit/puppet/type/registry_key_spec.rb' file. As a result, there
are no more deprecation warnings from Rspec.